Fish processing operations typically include removal of the heads and the fish entrails. Subsequently, the fish is split longitudinally and vertically into three pieces, two side-pieces, fish fillets, and one intermediate “carcass” piece containing the backbone. In the following context, the anatomy of the fish will be referred to as follows. The spine part of the fish is the part extending upwardly from the backbone to the upper back of the fish. The abdomen part of the fish is the part extending downward from the backbone to the abdomen or belly. The centre plan is a longitudinally extending vertical plan intersecting through the middle of the backbone in the middle in between the two fillets. The fillets are the pieces of meat arising when the fish is split as described above, i.e. longitudinally and vertically along each sides of the backbone thus splitting the fish into two fillets (side-pieces) and one carcass part comprising the backbone. The carcass is merely used for industrial purposes, e.g. for making fishmeal etc. while the two side-pieces represent the valuable part of the fish and may be further processed, e.g. by smoking.
In general, the process of filleting fish is widely automated. In existing machines, one or more knife arrangements cut the fish into fillets either by cutting from one side of the fish throughout the fish or by cutting partly from the abdomen side and partly from the spinal side of the fish. Most frequently, filleting machines are provided with one or more sets of two adjacent circular rotating saw blades. Obviously, the rotating saw blades perform a plan cut into the fish. In order not to loose too much fish meat, some machines are provided with a set of disks being arranged to form an angle therein between, so that the blades in one end defines a relatively open gab therein between and in a diametrically opposite end a relatively narrow gab therein between. In that way, the set of two blades can cut a triangular carcass part out of the fish relatively narrow to the backbone. However, mainly due to the fact that each blade performs a plan cut, relatively large amounts of the meat, especially in the abdomen part of the fish (the part extending downward from the backbone), is cut off from the valuable fillets and removed with the valueless carcass part.
New developments have generated machines using relatively flexible rotating band saws, i.e. closed-loop belts with a very sharp front edge. These belts are characterised by allowing cutting in slightly curved shapes. During the cutting, a fish is advanced in between two band saws. When the backbone has entered in between the two band saws, the two bands are pressed towards each other, e.g. by means of squeezing nippers. Due to a combination of the stiffness of the backbone and the angle of the knife edges of the bands, each of the bands will follow the backbone smoothly. The widest gab between the bands exists where the bands are held separated by the backbone. From this point, the bands, both upward towards the spine part and downward towards the abdomen part, follow a curve so that the gab between the bands, near the back of the fish and near the belly of the fish, is almost eliminated. The squeezing nippers are typically arranged above and below the fish (in close vicinity to the fish). Due to the curved configuration of the cutting bands, the cutting may be performed closely around the backbone and in a curve towards the centre plan of the fish. Accordingly, it is possibly with a band-saw machine to fillet the fish with a larger amount of the meat on the fillets.
However, even with the band-saw machines, a fair amount of meat is disposed with the carcass. The main reason being that the two band saws can not be pressed closer together than the width of the spine of the fish.
GB 973.213 describes a method and an apparatus for filleting fish, and wherein the fish, in particular herrings, is filleted by using firstly a rotating knife for cutting an incision in the spine part of the fish. Then the side fillets are cut away from the carcass by use of two oppositely arranged band saws. However, this single knife can only be used for fish having hair-like fishbone in the spine part. The way of using only one knife does not allow for cutting the fillets free on both side of fishbone in the spine part, which is necessary when filleting fish with large, thick and stiff fishbone in the spine part, such as Salmonidae or Gadidae. Further, it is not possible to cut away the dorsal fin from the spine part by using one knife, as the bone of fin extending in to the spine part must be cut on each side in order to remove it from the fillet.
U.S. Pat. No. 4,534,085, U.S. Pat. No. 3,596,308 and WO 94/17670 all relates to apparatuses, wherein the fish is filleted by use of two oppositely arranged band saws cutting the fillets on each side of the carcass.
U.S. Pat. No. 2,479,010 discloses a filleting machine, wherein the head and tail is removed first, and the fillets are then cut away by use of two rotating knives.
GB 811.199 discloses an apparatus, wherein the fillets are removed from the carcass by use of two rotating knives positioned just above the fish, which is transported on a conveyor.